高分辨大尺寸图像的目标检测切图处理

对于yolo等目标检测框架，输入的尺寸通常为640x640，这对于常规的图片尺寸和常规目标检测足够了。但是在诸如航拍图像等任务上，图像尺寸通常几千x几千甚至上万，目标也是非常小的，如果resize到640的尺寸，显然目标都已经丢失完了。实际上可以通过切图的方式进行推理，也就是将高分辨率的图切成640x640的图像块，每一个块再去做目标检测，最后再将图像块中目标的坐标转换为大尺寸图上即可。

以下是基于python的切图：

import torch
from PIL import Image
import numpy as np
import math
import matplotlib.pyplot as plt
import cv2

def split_and_overlap(image_path, output_size=640, overlap_pixels_x=100, overlap_pixels_y=100,normalize=False):
    """

    @param image_path:
    @param output_size: 切图小图的尺寸
    @param overlap_pixels_x: 经过在训练集上可视化分析，目标框的长和框都分布在100像素内，故设为为100像素
    @param overlap_pixels_y:
    @return:
            (num_cut, Channel, output_size, output_size)=》(切图个数，通道，小图尺寸),
            {"num_x": 在x轴上的切图数量，
             "num_y": 在y轴上的切图数量}

    """
    # 打开图像 1281*1920*3
    original_image = cv2.imread(image_path)
    if normalize:
        original_image=original_image/255
    original_height,original_width,_ = original_image.shape

    # 计算每个小图的大小
    tile_width = output_size
    tile_height = output_size

    # 初始化结果列表
    result_images = []
    
    # 计算能够整切的图像的长和宽
    target_width = math.ceil((original_width - overlap_pixels_x) / (output_size - overlap_pixels_x)) * (
            output_size - overlap_pixels_x) + overlap_pixels_x
    target_height = math.ceil((original_height - overlap_pixels_y) / (output_size - overlap_pixels_y)) * (
            output_size - overlap_pixels_y) + overlap_pixels_y

    # 填充到target尺寸
    original_image_pad = np.pad(original_image, ((0, target_height-original_height),(0, target_width-original_width), (0, 0)), mode='constant', constant_values=0)

    # 开始切割和堆叠
    for y_block_id in range(0, (target_height - overlap_pixels_y) // (output_size - overlap_pixels_y)):
        for x_block_id in range(0, (target_width - overlap_pixels_x) // (output_size - overlap_pixels_x)):
            # 裁剪图像
            left = max(x_block_id * (tile_width - overlap_pixels_x), 0)
            up = max(y_block_id * (tile_height - overlap_pixels_y), 0)
            box = (left, up, left + output_size, up + output_size)
            print(box)
            tile_image = original_image_pad[up:up + output_size,left:left + output_size,:]

            # 添加到结果列表
            result_images.append(tile_image)
	# 将result_images转为ndarray，形成[num_block,3,630,630]的张量
    return (np.array(result_images).transpose((0, 3, 2, 1)),
            {"num_x": (target_width - overlap_pixels_x) // (output_size - overlap_pixels_x),
             "num_y": (target_height - overlap_pixels_y) // (output_size - overlap_pixels_y)})


def visualize_tensor(tensor, columns=3):
    # 获取张量的形状
    batch_size, num_channels, height, width = tensor.shape

    # 计算行数
    rows = int(np.ceil(batch_size / columns))

    # 设置画布大小
    plt.figure(figsize=(15, 15))

    # 循环遍历每个图像
    for i in range(batch_size):
        plt.subplot(rows, columns, i + 1)

        # 提取图像数据
        image_data = np.transpose(tensor[i], (2, 1, 0))  # 将通道放在最后的顺序

        # 可能需要进行适当的缩放或预处理，具体取决于你的数据

        # 显示图像
        plt.imshow(image_data)
        plt.axis('off')  # 关闭坐标轴
    plt.subplots_adjust(wspace=0.05, hspace=0.05)
    plt.show()


if __name__ == "__main__":
    # 示例用法
    image_path = "yolov5/0006.jpg"
	
	# 切图
    result_images, cut_meta = split_and_overlap(image_path,normalize=False)

    for i in range(result_images.shape[0]):
        # 获取单张小图片的数据
        image_data = result_images[i].transpose(2, 1, 0)  # 将通道放到最后一个维度
        image = Image.fromarray(image_data)

        # 保存小图片，方便验证
        image.save(f'cut/image_{i + 1}.png')


    print(cut_meta)
    inp = torch.tensor(result_images, dtype=torch.float32)
	
	# 可视化小图片
    visualize_tensor(inp,columns=cut_meta['num_x'])

通过将[B,3,640,640]传入yolo检测中，相当于同时对B张图片进行检测，最终得到B个图像中的检测框信息。那么如何将小图上的检测框bbox坐标转换到大图上的坐标呢？也很简单

对于某一个小图，指导在B维度上的序号idx就知道这是第几张小图，然后通过列数运算能够得知所处几行几列的位置：

# 前面有N行,M列
N = cut_block_id // num_block_x
M = cut_block_id % num_block_x

# 也就是左侧有M个540像素，上侧有N个540像素，

# 在整图上的绝对坐标
center_x = center_x + M * 540
center_y = center_y + N * 540

如图所示，某个小图中坐标为(50,90)，通过计算得知所处第5列，则左侧有4个小图，考虑到有重叠区域，则左侧有4个540像素，故该坐标在大图中侧横坐标为50+540x4=2210。同理，也可以通过这样的方法计算其在大图中纵坐标的位置。